1. Field of Invention
The present invention relates to an optical head and an optical disk apparatus, and more particularly to an optical head and an optical disk apparatus, whose beam spot is rendered minute.
2. Description of Related Art
In optical disk apparatus, realization of both higher density and larger capacity of optical disks from compact disks (CD) to digital video disks (DVD) is proceeding, and optical disk apparatuses are increasingly requested for larger capacity in order to meet the tendency of computers toward higher performance and that of displays toward higher definition.
The recording density of an optical disk is basically limited by the size of a beam spot formed on a recording medium. When light is condensed by an objective lens, a diameter (beam spot size) D.sub.1/2 at which the optical intensity of the beam spot becomes 1/2 is given by the following equation (1), and the track width becomes substantially equal to this size. EQU D.sub.1/2 =k.lambda./(n.multidot.NA) (1)
where k: Proportionality constant (normally about 0.5) depending on the intensity distribution of the beam,
.lambda.: Wavelength PA1 n: Refractive index (normally air, nearly 1) of medium at the position of beam spot, PA1 NA: Numerical aperture of the objective lens. PA1 NAo: NA of incident light on SIL 54
Since the NA of objective lenses used with conventional optical disks is about 0.5, D.sub.1/2 is nearly equal to the wavelength. Also, as can be seen from equation (1), the use of a shorter wavelength or objective lenses of larger NA is effective to obtain a minute beam spot, and development efforts have been made respectively. In DVDs, the wavelength was shortened to 0.65 .mu.m, and the NA of the objective lens was raised from 0.45 in the case of CD to 0.6 thereby providing a density roughly four times higher than CD in DVD. As for the wavelength, a green or blue luminous has further been vigorously developed. On the other hand, as for NA, when it exceeds 0.6, the influence of signal intensity fluctuation due to tilt of the optical disk becomes significant. Thus, it is difficult to increase the NA higher than 0.6 in the conventional optical recording system which is performed using a plastic substrate. Therefore, current optical storage development is shifting toward condensing light on a recording layer formed on a plastic substrate without passing the light through the plastic substrate.
In the optical recording systems that directly condense light on a recording layer, the following two systems using near field optics have been recently proposed for radically reducing the beam spot size. These systems have been both obtained by applying the high-resolution techniques of microscopes to optical recording.
The first system employs near field optics for recording in which light is emitted from the tip end of an optical probe whose tip end has been polished to a small tapered shape (several tens of nanometers or less). This system has many problems such as difficult and unstable working of the probe, susceptibility of the probe to mechanical shocks, short life, low light utilization efficiency of 1/1000 or less, and requires many improvements to put it to practical use.
The second system places a hemispherical lens (Solid Immersion Lens (hereinafter, abbreviated to "SIL")) consisting of a transparent medium having a high refractive index near the focus of an objective lens to thereby form a minute beam spot at the central portion of the bottom of the SIL for performing optical recording, and can be considered to be a technique having comparatively higher feasibility than the first system. Since the wavelength of light becomes shorter in inverse proportion to the refractive index of the SIL within it, the beam spot also becomes smaller in proportion thereto. The majority of light condensed at this beam spot is totally reflected toward the hemispherical surface of the SIL, and some portion thereof is emitted in the neighborhood of the beam spot outside of the SIL as near field light. If a recording medium having nearly the same refractive index as the SIL is arranged in the neighborhood (at a sufficiently smaller distance than the wavelength of light), the near field light enters this medium and propagates within the medium. By using this light to record on the medium, it becomes possible to perform high-density recording. Since, however, an aberration of the objective lens remains present, it is necessary to maintain the aberration of the objective lens sufficiently low. The light condensing system using this SIL has two types to be described below.
FIG. 13 shows an optical head of the first type. This optical head 50 comprises an objective lens 52 for condensing a collimated beam 51, and a hemispherical SIL 54 arranged so that a bottom face 54a thereof intersects convergent light 53 from the objective lens 52. When the collimated beam 51 is incident on the objective lens 52, the collimated beam 51 is condensed by the objective lens 52, the convergent light 53 from the objective lens 52 is incident on the hemispherical surface 54b of the SIL 54, and is condensed at the center of the bottom face 54a of the SIL 54 to form a beam spot 55. The diameter of the beam spot 55 at the optical head 50 is reduced in inverse proportion to the refractive index of the SIL 54. When the recording medium 56 is brought close to the beam spot 55, the near field light in the neighborhood of the beam spot 55 is incident on the recording medium 56 as propagation light.
FIG. 14 shows an optical head of the second type. This optical head 50 comprises an objective lens 52 for condensing a collimated beam 51, and a bottomed SIL 54 arranged so that the bottom face 54a thereof intersects convergent light 53 from the objective lens 52. The SIL 54 is arranged so as to refract the convergent light 53 from the objective lens 52 and further condense it. The SIL 54 is constructed such that the collimated beam 51 is condensed in a distance of r/n (r is radius of SIL) from the center 54c of the hemispherical surface 54b (called "Super SIL Structure"), whereby it is possible to have small spherical aberration due to the SIL 54, to raise the numerical aperture within the SIL 54 to n times that of the objective lens 52 shown in FIG. 13, and further to make the beam spot 55 minute. That is, the beam spot can be rendered minute as shown by the following equation (2): EQU D.sub.1/2 =k.lambda./(n.multidot.NAi)=.lambda./(n.sup.2.multidot.NAo) (2)
where NAi: Numerical aperture within SIL 54
However, NA of the incident light on this Super SIL 54, that is, the maximum value .theta.max of the incident angle .theta., is inversely related to the refractive index n of the SIL 54, and the two cannot be made independently large.
FIG. 15 shows the relationship between the refractive index n of Super SIL 54 and NAo, obtained by Suzuki in #0C-1 of Asia-Pacific Data Storage Conference (Taiwan, '97, 7) (hereinafter, referred to as "First conventional example"). As can be seen from FIG. 15, when the refractive index n of the SIL is continuously raised, the maximum value NAomax which the NAo of the incident light can take gradually becomes smaller. This is because when the NAo increase s over the maximum value NAomax and the incident angle becomes larger, the beam spot 55 at the position of the recording medium 56 becomes wider because the light does not pass through the SIL 54, but becomes directly incident on the recording medium 56. When, for example, the refractive index n=2, NAomax is 0.44, and the product n.multidot.NAomax is within a range of 0.8 to 0.9. This is the theoretical limit, and in reality is a smaller value (0.7 to 0.8).
Concerning the condensing experiment using the Super SIL, B. D. Terris et al reported in Appl. Phys. Lett. Vol. 68 ('96), P. 141 (hereinafter, referred to as "Second conventional example"). According to this report, a Super SIL having a refractive index n=1.83 is placed between an objective lens and recording medium, and a laser beam with a wavelength of 0.83 .mu.m is condensed to thereby obtain a beam spot size of 0.317 .mu.m. In other words, condensing equivalent to D.sub.1/2 =.lambda./2.3 is accomplished, and in this case, NA is 0.4, and n.multidot.NAmax is about 0.73. Also, the possibility of recording density (3.8.times.10.sup.8 bits/cm.sup.2) several times the conventional density has been verified using this system.
FIG. 16 shows an optical disk apparatus (hereinafter, referred to as "Third conventional example") described in the specification of U.S. Pat. No. 5,497,359. This optical disk apparatus 500 comprises: an optical disk 501 obtained by forming a recording layer 501b on a plastic substrate 501a; a motor 504 provided on a base 502, for rotationally driving the optical disk 501 through a shaft 503; a flying slider 505 consisting of a transparent medium, for levitate-traveling on a recording layer 501b of the optical disk 501; a hemispherical SIL 54 mounted to the flying slider 505; a detection optical system unit 510 for generating signals for automatic focusing control and tracking control, or data signals from light reflected by an optical system for shaping and condensing a beam from a semiconductor laser, and the optical disk 501; an arm 506A for supporting the detection optical system unit 510; an arm 506B mounted to the arm 506A, for supporting the flying slider 505; and a voice coil motor (VCM) 507 provided on the base 502, for driving the arm 506A to cause the SIL 54 and the detection optical system unit 510 to access and track at the same time.
FIG. 17 shows the detail of the SIL 54 and the flying slider 505 of the third conventional example. The flying slider 505 is formed of a transparent medium having nearly the same refractive index as the SIL 54. The flying slider 505 is fixed to the hemispherical SIL 54, and a laser beam is condensed on the lower surface of the flying slider 505 to form a beam spot 55, whereby the Super SIL is constituted of the flying slider 505 and the SIL 54.
FIG. 18 shows the detail of a detection optical system unit 510 according to the third conventional example. In this detection optical system unit 510, the conventionally most general optical system is adopted, and this system is not improved so as to particularly conform to the SIL 54. More specifically, the detection optical system unit 510 comprises: a semiconductor laser 511 for emitting a laser beam 511a; a collimator lens 512 for collimating optical-power output 511a from the semiconductor laser 511 into a collimated beam 511b; a beam splitter 513 for splitting the optical-power output 511b from the semiconductor laser 511 and light reflected by an optical disk 501; a mirror 514; an objective lens 516A that is driven by an actuator 515, and condenses a collimated beam 511c from the semiconductor laser 511 on the optical disk 501; a photodetector 517 for detecting the light reflected by the optical disk 501, split by the beam splitter 513, and focused by a lens 516B; and an amplifier 518 for amplifying data signals (DAT) or control signals (FES, TES) that are output from the photodetector 517. For the SIL 54, a lens having a diameter of 2 mm is used, but in this case, the beam size at the position of the objective lens 516A is about 4 mm. Therefore, each optical system components 512, 513, 514, 516A and 516B in the detection optical system unit 510 require an effective aperture of 4 mm, nearly the same as the beam size, or larger.
Also, the optical disk apparatus 500 performs tracking control by means of one-stage control using a VCM 507 alone, and automatic focusing control for driving the objective lens 516A by an actuator 515. Since the depth of focus decreases inversely with the square of NA or the cube of n, the depth of focus in the case of condensing using the SIL 54 is as small as 0.2 .mu.m or less. On the other hand, since there is a convergent beam between the objective lens 516A and the SIL 54, the depth of focus expands and contracts due to temperature fluctuations that cause focal displacement. Further, since temperature causes the laser wavelength to fluctuate, focal displacement additionally occurs due to chromatic aberration of the objective lens 516A. For this reason, highly precise automatic focus control is necessary to reduce the above-described focal displacement.
In an optical disk apparatus using an SIL that is levitated in proximity and travels on the recording medium, the optical head is suitable for applications in which the optical disk is used as a fixed and non-replaceable one, which is similar to a conventional magnetic hard disk. For this reason, it becomes indispensable to have high volumetric recording density when disks are stacked to be multi-head and multi-disk as well as a large recording capacity and high data transfer rate. In the case of the latest hard disk, since the disk interval is 3 mm or less, it is necessary to reduce the height of the optical head to that of the head (about 2 mm or smaller) of the hard disk.
FIG. 19 shows an optical head (hereinafter, referred to "Fourth conventional example") described in the specification of U.S. Pat. No. 5,497,359, developed to cope with such a need for miniaturization of the optical head. This optical head 50 is obtained by making an SIL 54, an objective lens 516A, a semiconductor laser and a detection optical system integral with one another on a flying slider 505. In FIG. 19, the semiconductor laser and the detection optical system are collectively shown as a single block 520, which is mounted to the flying slider 505 by a mounting member 521. Since the distance between the objective lens 516A and the block 520 is made shorter to thereby reduce the influence of change in temperature, there is no need for any automatic focusing control mechanism.
On the other hand, a conventional optical head whose weight has been reduced is shown in "Digest of Optical Data Storage ('93) P. 93" (hereinafter, referred to as "Fifth conventional example"). This optical head adopts a separation-type optical system in which the semiconductor laser and the detection unit are separated from the objective lens unit and fixed, and in which only the objective lens portion is caused to travel by a VCM. Tracking is performed by means of two-stage control in which the VCM is used for tracking in a low frequency area, while a galvano-mirror is used for tracking in a high frequency area. Thus, it is possible to reduce the weight of the movable portion including the objective lens to 7 g including the VCM. Also, it is possible to enlarge the frequency band up to about 30 kHz (gain of about 80 dB) by means of tracking using two-stage control.
FIG. 20 shows a conventional optical disk apparatus (hereinafter, referred to as "Sixth conventional example") described in the literature "The Nikkei Electronics Journal (No. 699, P. 13, '97.9.22)". This optical disk apparatus 500 has a separation optical system using a galvano-mirror for tracking, and comprises: a flying slider 505 for levitate-traveling on an optical disk 501; an SIL (not shown), an objective lens 530 and a folding mirror 531, which are mounted on the flying slider 505; an arm 532 for supporting the flying slider 505; a VCM 533 for driving the arm 532; a fixed optical system 534; and a mirror 535 for directing light from the fixed optical system 534 to the objective lens 530. The adoption of the separation optical system makes it possible to reduce the weight of the movable portion and to enlarge the frequency band for tracking as in the case of the fifth conventional example.
According to the first and second conventional examples, however, there is a problem that since the refractive index n of the SIL is reciprocally related to the maximum NAmax, the theoretical limits of the product n.multidot.NAmax are 0.8 to 0.9, so that the beam spot size is large and higher density cannot be achieved.
Also, in the optical disk apparatus 500 of the third conventional example, since the SIL 54 uses a lens with a diameter of 2 mm, a beam size of about 4 mm is required. Further, since the objective lens 516A must have a low chromatic aberration, the lens size (diameter or height) is large and makes the optical system large. Also, since a convergent beam is used for the beam incident on the SIL 54, an automatic focusing control mechanism is required because the convergent point varies depending on temperature fluctuations. Accordingly, the weight of the optical head is as heavy as 10 g or more, the height is as high as about 10 mm, and the intervals at which the optical disks 501 can be stacked are large, and therefore, leads to a problem that the volumetric capacity cannot be made small enough as that of the magnetic hard.
More specifically, it is possible to reduce the size of the entire optical system by making the diameter of the SIL 54 smaller, but the thickness of the flying slider 505 must be made thinner at the same time, and therefore, there is a limit to how small the system can be made. Namely, the thickness of the flying slider 505 becomes substantially equal to a distance r/n between the center and the condensing point, and if a medium having a refractive index of 2 is used, when a radius of the SIL 54 is 0.5 mm, the thickness of the flying slider becomes 250 .mu.m, which is a minimum thickness to maintain mechanical strength.
In addition, when the weight of the optical head exceeds 10 g, there is a problem that high-speed tracking cannot be performed, nor can the data transfer rate be raised.
In the optical head 50 of the fourth conventional example, since it is actually difficult to miniaturize the optical head 50 and an automatic focusing control mechanism is required, there is a problem that the height of the optical head 50 increases, and it is difficult to make the device small as in the case of the third conventional example.
Also, although it is considered that no automatic focusing control is required, the optical system is susceptible to expansion and contraction of the optical system supporting member due to temperature fluctuations. In addition, it is also necessary to correct focal displacement due to fluctuations of the laser wavelength, making it difficult to eliminate the automatic focusing control mechanism.
The optical head in the fifth conventional example has a problem that high data transfer rate cannot be attained because the galvano-mirror has limits in the high frequency area.
More specifically, the track width becomes narrower as the beam spot is made smaller, and accordingly higher-speed and higher performance tracking control is required. The track width has a size equal to nearly 70% of an ordinary beam spot size D.sub.1/2 as seen in a DVD. Accordingly, when the beam spot size D.sub.1/2 is 0.31 .mu.m, the track width is 0.2 .mu.m, and when a blue laser (410 nm) is used, the track width is 0.1 .mu.m or smaller. On the other hand, tracking must normally be performed at a tolerance of about one tenth of the track width, in other words, tracking with precision of.+-.0.01 .mu.m is required. Also, since tracks have been formed by stamping in advance in the optical disk, a decentered track with.+-.several tens of microns occurs during the process. In order to track this track at a tolerance of.+-.0.01 .mu.m, a tracking error of.+-.0.01 .mu.m is detected to follow up with.+-.several tens microns, and the control system requires a gain of more than 80 dB. Also, since the tracking control system is a secondary system and the band expands at -40dB/decade, the rotary speed is set to 3,600 rpm, and, a frequency band of about 200 kHz is required to perform tracking of 0.01 .mu.m. More specifically, the band is for 30 kHz even when the galvano-mirror is used as discussed above, and it is difficult to perform tracking using one-stage control. It is necessary to reduce the rotary speed by one or more orders of magnitude, or to use a driving mechanism having lighter weight and higher performance than the galvano-mirror. A higher data transfer rate as well as higher density is naturally requested, but decreasing the rotary speed means a decrease in the data transfer rate in proportion thereto, which is a problem.
Also, in an optical disk apparatus 500 of the sixth conventional example, since the beam size has been set to 4 to 5 mm in order to reduce beam positional aberrations resulting from the movement of the optical head, the height of the optical head is nearly 10 mm. Thus, the intervals at which the optical disks 501 are stacked must be relatively large, and it is difficult to make the apparatus small in size. Also, although the wavelength is 680 nm, about 20% shorter than in the above-described example, the track pitch is set to 0.34 .mu.m, which is designed to be larger than the theoretical value 0.2 .mu.m of the spot size in this system, and the advantage of the SIL is not fully utilized.
The respective problems described above result from the fact that the SIL alone is not capable of condensing sufficiently, but two-stage condensing by a combination with an objective lens is required, and are essential problems of the optical head using the SIL.
Therefore, it is an object of the present invention to provide a small-sized optical head and optical disk apparatus capable of high-density recording with an improved data transfer rate.